In computer systems, two components or devices, for example, a motherboard and an internal hard disk or an external peripheral device, connect to one another by physical interfaces to be able to interchange data with one another. Popular interfaces are the Universal Serial Bus (USB), the Small Computer System Interface (SCSI), the Serial Attached SCSI (SAS), the Serial Advanced Technology Attachment (SATA) or the Fiber Channel (FC), for example. The transmission properties such as the transmission speed, the form and content of individual data packets or the addressing and communication among connected devices, are stipulated for each interface in an associated transmission protocol.
A physical interface comprises cables, electrical conductors and plug connections between the individual devices. The data and information are transmitted between the devices by analog signals. Signal transmission on the transmission link of the interface usually takes place on two differential signal lines. An analog signal is transmitted in unaltered form on one line (“+”) and in inverted form on the other line (“−”). Finally, the two lines + and − form a transmission link. The signals are modulated in terms of their amplitude, for example, to encode digital information from individual bits. In the case of the + line, for example, a low level in the voltage corresponds to the logic value “0” and a high level in the voltage corresponds to the logic value “1”. On account of the inversion of the − line, the converse case is obtained for the − line.
Differential transmission of a piece of information by signals on the lines + and − has the advantage that a difference signal based on “+”−“−” for the two signals can be formed. The voltage difference between the low level and the high level of the difference signal formed is therefore twice the magnitude of the single ended signal. Irradiated distortions can be largely eliminated which means that transmission certainty can be increased.
Each device has a transmitter and a receiver which send and receive analog signals via the interface in accordance with a protocol. In that case, the transmitters and receivers are specific circuits or integrated chips (ICs), and the transmitter and the receiver in a device are often described together as what is known as a “transceiver.” The individual circuits of the transmitters and receivers are also called “transmitter-PHY” or “receiver-PHY” based on their task of physical signal transmission.
A physical interface with its transmission link comprising connectors, lines and conductor traces on boards influences the signal transmission between a transmitter and a receiver in two devices. The reason is that the transmission link is real and, hence, not ideal and has negative properties. On account of high-frequency scatter, electromagnetic interference, crosstalk or reflection (to name just a few negative properties), transmitted signals are altered and disturbed on the physical interface during transmission. By way of example, fluctuations in amplitude, phase (phase noise or jitter), frequency, slew rate or a combination of these effects arise for the transmitted signals.
In particular, a mismatch or imbalance may occur between the individual signals on the + and − lines if the transmission link has imbalances between the two + and − lines or negative effects arise with an imbalance in the + and − lines. The signals may thus also have an imbalance in terms of amplitude, phase, frequency, slew rate or a combination of these effects. This can result in impairment of the signal quality of the received signals (reduction of the horizontal and vertical eye opening in an eye diagram), in an increased bit error rate (BER) or in the worst case in loss of information.
Usually, properties of the transmitted transmission signals are asserted once on the basis of the transmission protocol used and are not changed again. Therefore, the parameters of a receiver are merely matched to the specification requirements on the basis of the prescribed transmission protocol. Matched parameters are stored in control registers in the receiver, for example. In the case of SAS technology, for example, matching can be performed during what is known as the “speed negotiation phase” (phase in which the transmission speed is determined) during a training phase. This receiver-end matching makes only a small contribution to improving the transmission properties, however.
US 2010/0208780 A1 discloses a signal transmission system, wherein data are transmitted differentially from a transmitter to a receiver. In the receiver, a DC component is ascertained from the differential signal and compared with a reference value. From this, a feedback signal is finally formed which is returned to the transmitter for the purpose of matching parameters in the transmitter. This solution allows only limited matching of the transmitter to disturbing effects within the transmission link.
It could therefore be helpful to provide a method and an arrangement of the type cited at the outset which allow improved matching of the signal transmission between two electronic devices.